1. Field of the Invention
The invention relates to a circuit arrangement for the dynamic control of -ceramic solid-state actuators, such as for example, piezotranslators with energy recovery by means of magnetic intermediate stores as well as a control loop for operating a piezotranslator.
2. Description of the Related Art
Piezotranslators are electrically controllable actuators whose functions can be attributed to the piezoelectric effect. Active sensors, so-called actuators, can perform the most delicate positioning movements with high accuracy from the subnanometer up to the millimeter range.
Under electrical aspects, a piezotranslator represents a capacitor whose charge has a proportional relationship to its expansion. Consequently, piezotranslators take up energy during the expansion process only. The expansion is maintained without further energy supply. Due to the high capacitance of the piezotranslators, however, a high output power of the driver circuit is required in the case of fast positional changes as they occur under dynamic operation conditions. The associated control electronics must therefore have special properties for each application case and has to be optimised in order to ensure the successful employment of a piezotranslator.
In control processes for the dynamic piezocontrol in which the actuating element must follow up fast changes of a reference variable, it is desired that the amplitude of the movement characteristic agrees with the input signal as close as possible. However, such a linear transfer behaviour cannot be ensured for frequencies up to any magnitude, but is limited, on the one hand, by the resonance frequency of the translator or the entire actuating system, respectively, and by the output power of the amplifier, on the other hand.
Due to the hysteresis phenomena of a piezotranslator the absolute expansion of the actuating element can be determined only inaccurately via the applied amplified input voltage of the amplifier. The resulting expansion has an inherent error of up to 10%, both with respect to its absolute value and to its relative movements. In order to eliminate this error, it is known to provide closed control loops, i.e.. a measuring system for the expansion and control electronics which control the operating voltage in accordance with a comparison of the reference variable and the actual value. Closed positioning control loops are therefore provided with external probes in order to be able to determine the position.
Due to the fact that piezotranslators can electrically be described as capacitors, as mentioned above, onto which electric charges must either be applied or from which said charges must be withdrawn for the purpose of a length variation, which for example can be realised by means of a switch, charging or discharging between the piezotranslator, on the one hand, and the charging current circuit, on the other hand, will inevitably result in power losses, if this is done via controllable variable resistors,. e.g. transistors.
From the final report of the joint project xe2x80x9cEntwicklung leistungsoptimierter, hybrider Hydraulikkomponenten auf der Basis piezoelektrischer Aktuatorenxe2x80x9d (Development of Power-Optimised Hybrid Hydraulic Components on the Basis of Piezoelectric Actuators) of the Institut fxc3xcr Fertigungstechnik und spanende Werkzeugmaschinen (Institute of Manufacturing Engineering and Metal Cutting Machine Tools), Hanover, Germany, November 1996, it is known to design digital control amplifiers for driving piezoelectric actuators, which are provided with a controlled energy recovery capacity. The known final stage preferably includes inductively coupled coils in order to increase the efficiency of the energy recovery.
In the known circuit arrangement two separate magnetic intermediate energy stores are provided, with the energy stores being switched by a controller in a clocked manner, in order to achieve a predetermined output voltage curve of the control circuit. At the piezotranslator a voltage-dependent non-linear charging of the capacitance will result, with the available current decreasing upon reaching the supply voltage. Due to the circuit arrangement as two separate blocking transformers, only one direction each of the output current can be driven.
In order to obtain a desired linear voltage increase at the piezotranslator, the storage volume of each store must be designed extremely large in the state of the art. The reason of this is that the blocking transformer must be dimensioned according to the smallest voltage increase AU at the range limits of the operating voltage. In the middle output range, however, the available storage volume cannot be utilised so that a correspondingly implemented output amplifier does not operate efficiently.
With respect to the control of piezotranslators in actual applications, the actual momentary value of the output voltage supplied to the piezotranslator is determined by means of a control loop. This is to compensate for existing deficiencies of the control circuit or the final stage, respectively, such as its non-linearity, temperature drift, and frequency dependence, as well as any undesired behaviour of the connected piezotranslator because of the already mentioned voltage hysteresis.
In this context, it has been known to apply part of the piezo output voltage as the actual value to the input of an error amplifier, or to use an exact physical measuring system which yields an output voltage as the actual value. The desired reference variable is supplied to the input of the error amplifier, with the output of the amplifier being connected to the control circuit itself.
The above described controller concept achieves satisfactory results with arrangements without energy recovery, which are mostly equipped with a conventional loss-inherent final stage of the type of controlled series resistors. However, problems occur with a corresponding use in energy recovering control circuits. The reason for this is the considerable inductances which are responsible for the desired energy recovery and which are connected in series to the piezotranslator. Together with the piezotranslator which represents a capacitor these form a high-quality series or resonance circuit. Depending on the type or size of the translator and employed inductor, its resonance frequency is mostly within the frequency band in the range from 1 to 5 kHz, which is of interest for the amplifier operation. The series resonance circuit in turn causes a high increase of the amplification in the resonance range, together with an undesired phase shift in the working band of the control loop approaching the critical 180xc2x0 limit, which affects the compensation. From this, an undesired post-pulse oscillation or self-oscillation results. The desired flat amplitude characteristic of the overall system up to the upper working frequency range is therefore no longer achievable.
Therefore, it is the object of the invention to provide a circuit arrangement for the dynamic control of piezotranslators with energy recovery as well as an improved control loop for the operation of piezotranslators, which allow the almost linear charging of the piezotranslator over the entire voltage range and at the same time the optimisation of the energy recovery with a small installation size of the implemented circuit. Simultaneously, energy storing elements such as capacitors or accumulators must be able to be operated at a maximum piezotranslator supply voltage so that the return currents can be maintained correspondingly low. With respect to the control loop, it is essential to prevent points of resonance in the working and transfer range so that any self-oscillation can effectively be avoided.
The object of the invention is solved in that for achieving a predetermined linear voltage characteristic at the piezotranslator, the secondary circuit is designed as a half-bridge consisting of the clocked switches at whose output the inductive intermediate store is arranged in series with the piezotranslator, with the clocked switches being externally controlled and operated at a high cycle or switching frequency in such a manner that the intermediate store is alternately connected with an upper or lower supply voltage at the most, with the series connection of piezotranslator and inductive intermediate store carrying a superimposed bridge direct current as well as, as far as the closed loop control is concerned, by a third controller for a positioning control, at whose first input the reference variable of the physical position of the piezotranslator and at whose second input a mechanical actual value which is detected via a sensor of the piezotranslator are applied, with the output of the third controller being connected with one of the inputs of the second controller, wherein preferably the second controller feeds back the integral of the piezotranslator current in lieu of a voltage which is proportional to the output voltage of the final stage. The remaining dependent claims represent at least suitable embodiments or developments of the invention.
The basic idea of the invention is to form the control circuit for the dynamic operation of piezotranslators as a half-bridge circuit with a single series coil as an intermediate energy store. In this case, the maximum available current for charging and discharging the piezotranslator is identical with the maximum current of the series coil. Such a limitation occurs unchanged and at a constant limit current over the entire range of the piezo output voltage. For the capacitance of the piezotranslator the constant limit current causes a constant voltage increase so that the storage capacity of the intermediate energy store is uniformly and fully utilised over the entire working range. The installation size can be reduced due to the use of a single series coil, with the indirect consequence of a cost reduction.
According to the invention, a direct-current superimposed single coil is employed as an intermediate energy store, with the coil to be considered as a forward converter under the aspect of its effect. The coil direct current flows during the entire activation period and is modulated by a high-frequency alternating current with a relatively small amplitude at the working cycle of the half-bridges, i.e. of the employed switches, at a frequency of essentially 100 kHz.
By means of the inventive arrangement of a single inductive magnetic intermediate store in the secondary circuit which is connected in series with the piezotranslator and with the secondary circuit being designed as a half-bridge, it is possible to significantly increase the efficiency of the power final stage formed in this manner. Due to the fact that there is no internal electrical isolation, the losses can be reduced further. Also otherwise present drawbacks due to voltage losses at the energy recovery diodes do no longer occur in a comparable magnitude. A transfer of the entire energy in the piezotranslator in each modulation wave to the primary side and back again as this is the case in the state of the art is no longer necessary.
Due to the fact that the only energy store is located on the secondary side and is subjected to the high piezo voltage which, depending on the translator type, is between 100 and 1200 V, the currents during the energy recovery can be kept low. With respect to circuit engineering, the only intermediate store can be arranged in a spatially close relationship with the piezotranslator so that electromagnetic radiated noise can be reduced to a minimum. A still better electromagnetic compatibility results from the effective modulation current which is superimposed by only a low high-frequency alternating current according to the external cycling of the half-bridges. By a corresponding dimensioning of the inductive intermediate store, the superimposed alternating current is in the order of essentially 10% of the modulation current so that the residual ripple will be greatly reduced.
Another basic idea of the invention is that when using MOSFETs as switches, the negative effects of existing internal inverse diodes can be avoided. For this purpose, the invention proposes to connect an external blocking diode in series with the clearance between open contacts and to bridge this series connection by an oppositely poled commutating diode. This additional diode combination prevents the inverse operation of the MOS transistors by its internal inverse diode and allows a quasi external inverse operation by the commutating diode which is oppositely poled with respect to the external blocking diode.
The additional diodes have a shorter recovery time relative to the MOS transistor in order to considerably improve the operation of the modified half-bridge circuit under the aspect of the switching frequency.
In the inventive control concept and according to another basic idea of the invention, a current sensor for determining a control voltage which is proportional to the output current is arranged for controlling the piezotranslator. This current sensor is connected with the input of a first controller, with the second input of the first controller being applied to the output of a second controller at whose inputs a given reference variable corresponding to the physical position of the piezotranslator and the reduced output voltage is applied.
Accordingly, the control concept consists of two nested, separate control loops, i.e. an inner and an outer control loop. The inner control loop encompasses the control circuit proper, including a potentially critical point of resonance which is formed by the magnetic energy stores which are provided there in any form and the capacitive load of the piezotranslator.
By means of the outer control loop it is possible to achieve the characteristic of the output voltage which is defined by the input signal, i.e. the reference variable, of the position and expected with an amplified amplitude at the amplifier output.
Due to the fact that the inner control loop has already eliminated the point of resonance and the associated additional phase shift in the working frequency range, the outer control loop can be optimised in a simple manner. As a whole, the design of the circuit arrangement for controlling purposes offers an improved overall transfer behaviour. The amplitude characteristic is uniform over the entire frequency range, including the point of the critical LC circuit and drops without transition point. The phase characteristic has a phase margin of at least 50xc2x0 and is not critical, while it is possible to keep the time behaviour at the maximum system dynamics free from resonances and overshooting.
As explained above, the inner control loop operates by utilising a current proportional sensor signal, with a current transformer being provided for this purpose. This current transformer can be realised by a simple series resistor in the return line of the load current or by a simple transformer circuit. The inner control loop thus enforces a current of a predetermined magnitude at the output of the final stage or of the control circuit, respectively, which flows into the load and which can be controlled in its time characteristic by the specified reference variable. Any additional loads connected with the capacitive piezotranslator load, such as, for example, a storage coil, have no influence on the common load current and cannot cause resonance phenomena.
Due to the fact that the voltage which is normally building up at the piezotranslator is defined by the integral of the current, the now controlled current, however, no longer has a point of resonance, it is possible to keep the piezovoltage, too, free from resonances. The inductance of the intermediate store, which is introduced for energy recovery, is no longer detected by the control behaviour of the inner control loop as a resonance-generating component and thus eliminated to the outside.
In an embodiment of the invention, an additional positioning control of the system is performed according to another basic idea, in that a third controller is provided at the first input of which the reference variable of the physical position of the piezotranslator and at the second input of which a physical actual value of the piezotranslator, which is detected via a sensor, is applied, with the output of the third controller being connected with one of the inputs of the above mentioned second controller.
The dynamic behaviour of the control system with inner and outer control loop can be improved according to another basic idea of the invention in that the second controller feeds back the integral of the piezotranslator current instead of controlling an output voltage which is proportional to the reference variable. This alternative feedback can be activated in a frequency dependent manner.
In a preferred embodiment, the integration value is used at frequencies of essentially  greater than 10 Hz, and at frequencies of  less than 10 Hz, the mentioned voltage feedback is performed.
The described advantage of the improved pilot control also has a positive influence of those control loops in which an additional positioning control by means of a further controller is dispensed with. These advantages are particularly effective at the upper limit of the working frequency range where the loop amplification must be reduced because of the operational stability and is therefore too low for an effective error compensation.